Need advice about working offline

I am trying to work offline to revise slides for a talk in China.  I have
limited web access.  I tried to revise a file which was working six weeks
ago, and it produced the following error message:

! LaTeX Error: File `url.sty' not found.

It offered to download url.sty but I had no web access at the time.  It is
easy to give a minimal example.  This single line produces that error
message:

\documentclass{beamer}

Now I have gotten web access and downloaded url.sty. But I need to know if
this kind of thing is likely to happen again when I work offline. Will
previously working latex files start to need new downloads?  Is there some
way around it when I can't get on line?

Or should I just work in Word when I need to revise documents offline?

best, Colin
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